Code division multiple access (CDMA) is a signal coding scheme that allows multiple independent communications channels to occupy the same frequency band at the same time with overlapping signal spectra. To achieve this, the communication signal within each of the channels is modulated with a unique code (e.g., a pseudo noise code) that spreads the spectrum of the communication signal and that is uncorrelated to (e.g., orthogonal to) the codes used in the other channels. The lack of correlation between different channel codes allows each of the communication signals to be recovered from a composite receive signal by correlating the receive signal with a corresponding code. CDMA techniques are capable of providing a significant increase in communication system capacity within a given available bandwidth.
To maximize capacity in a CDMA based system, the power levels used in the various channels have to be substantially equal. This is because each of the channels appears as noise to each of the other channels during the correlation process. Thus, if the power level of signals in one of the channels is increased, the noise experienced by each of the other channels is correspondingly increased. This increase in noise will compromise accurate correlation in the other channels. Therefore, to reduce the noise in the other channels, the overall number of channels, and thus the capacity of the system, must be reduced.
Some communication systems, such as satellite-based systems, require power consumption to be minimized. These systems ordinarily achieve this power conservation by tailoring transmit power levels to the individual remote entities they are communicating with. For example, a communications satellite will generally communicate with a large number of terrestrial users simultaneously. Each of these users will have different power requirements due to, for example, varying levels of obstruction in the propagation paths between the satellite and the terrestrial users (i.e., different levels of shadowing). To communicate with users that are heavily shadowed, the satellite needs to transmit relatively high power levels. To communicate with slightly shadowed or non-shadowed users, the satellite can transmit relatively low power levels. If the satellite transmitted the same power level to all of the terrestrial users, more power would be expended on the slightly shadowed users than was necessary for accurate communication. Thus, in the past, a tradeoff was generally made between communications capacity and power efficiency in CDMA communications systems. Therefore, a need exists for a communication system that can efficiently process communications at various power levels. The communication system will preferably be capable of achieving an enhanced level of power efficiency without significantly reducing system capacity.